It is mind boggling to think of God as always having existed. He has lived forever and will still live as long as we do. This teaching can become problematic as we deal in terms of creationism versus theories like the Big Bang. One argument against the latter theory is that it is impossible for an actually infinite number of things to exist; also the second law of thermodynamics indicates that the universe is “winding down” from a point of higher usable energy. Is God subject to these laws?
In other words, how can one say God has always existed and that matter can't have always existed?
To answer this question, I have the help of two great theologians: C.S. Lewis and St. Augustine. Lewis argues in Mere Christianity that God's relationship to time is like the relationship between an author and her work.
“Almost certainly God is not in Time. His life does not consist of moments following one another...He has all eternity in which to listen to the split second of prayer... Suppose I am writing a novel... Between writing the first half of that sentence and the second, I might sit down for three hours and think.” (pp. 167-168; HarperSanFrancisco, 2001 edition)In my own words, God is not just on the entire timeline, he is outside the timeline. He created time itself. Therefore, to say God has always existed is an understatement. He invented both “existence” and the concept of “always.”
Augustine sheds more light on the issue in The City of God.
"...time does not exist without some movement and transition, while in eternity there is no change, who does not see that there could have been no time had not some creature been made, which by some motion could give birth to change...and thus, in these shorter or longer intervals of duration, time would begin? Since then, God, in whose eternity is no change at all, is the Creator and Ordainer of time, I do not see how He can be said to have created the world after spaces of time had elapsed, unless it be said that prior to the world there was some creature by whose movement time could pass. ... then assuredly the world was made, not in time, but simultaneously with time." (Book 11, The City of God)In other words, if nothing changes, there is no passage of time. Therefore, God did not "wait" for time to pass before creating the world. He created time by the simple act of creating. Time and the world began at the same...time.
Time itself is subject to God. Therefore, issues like God existing within creation for ever are subject to this greater principle. God's immanence is explained by his transcendence.